East Calcutta Wetlands (ECW), lying east of the city of Kolkata (formerly Calcutta), West Bengal in India, demonstrates the usage of city sewage for traditional practices of fisheries and agriculture. As a Ramsar Site, the wetland demands exploration of its bioresources for better understanding and management of the ecosystem operating therein. Butterflies (Lepidoptera: Rhopalocera) being potent pollinators and ecological indicators, are examined in the present study. The diversity study, conducted for two consecutive years (Jan. 2007-Nov. 2009) in all the three seasons (pre-monsoon, monsoon and post-monsoon), revealed seventy-four species. As butterflies depend on preferred host and nectar plants during their larval and adult stages respectively, the lack of these sources in some parts of ECW indicate degraded habitats with low species richness. Ongoing unplanned anthropogenic activities like habitat modifications (conversion of wetlands to agricultural lands) are resulting in the loss of wetland biodiversity and hence ecosystem integrity in ECW.